The traditional way to achieve optical large capacity storage is to store the data in three special dimensions of the medium, namely multilayer storage, or throughout volume of the medium, namely holographic storage. The main problems for the traditional multilayer and holographic storages are interlayer crosstalk noise and disk rotation blurring, which limits the available layer number and degrades the holographic recording quality, respectively.
Another way to achieve optical large capacity storage is the multi-dimensional storage, which adds extra “dimensions”, such as the physical dimensions of wavelength, intensity, polarization, phase and even photon angular momentum to three spatial dimensions to increase the storage capacity and improve the system performance. Several four-dimensional and one five-dimensional optical storages have been proposed. Two years ago, Shangqing Liu proposed a six-dimensional storage technology [Shangqing Liu, “Six-dimensional optical storage method and apparatus,” U.S. Pat. No. 8,503,279, Issued: Aug. 6, 2013], which adds three physical dimensions of wavelength, intensity and polarization to three spatial dimensions to get large storage capacity and excellent system performance. The theoretical research results show that its storage capacity is over 10 Tbytes per DVD sized disk with ultrafast data transfer speeds.
The extraordinary advantages of that six-dimensional storage are its relatively simple structure and easy light manipulation for addressing the desired storage cell in the fast rotating disk, which make an optical large capacity system of over 10 Tbytes per DVD size disk actually practicable, and a dramatic increase of the data writing, erasing and reading speeds.
In fact, a few proposed optical storage schemes promised larger capacities. An example is the multilayer storage utilizing super resolution writing, which is inspired by a diffraction-unlimited far-field imaging method [S. W. Hell and J. Wichmann, “Breaking the diffraction resolution limit by stimulated emission: stimulated-emission-depletion fluorescence microscopy,” Opt. Lett. Vol. 19, pp. 780-782, 1994]. This super resolution writing storage allows the created storage cell size down to nano scale, which is in theory equivalent to Pbytes (petabytes, 1 Pbyte=1024 Tbytes) to Ebytes (exabytes, 1 Ebyte=1024 Pbytes) storage capacity per DVD size disk. However, because the super resolution writing disk is bit-based, that is, each storage cell only holds 1 bit data, its ultrahigh storage capacity depends on its ultra small cell size, which results in serious technical obstacles to the realization of this technology in practice, which include difficult optical readout of nano scale data marks hindered by diffraction limitation, unacceptable low data transfer speeds to ultrahigh capacity as a bit by a bit writing and reading, awesome interlayer crosstalk noise due to 103 to 2×105 data layers just in a disk of 1 mm thick and too arduous light manipulation for addressing the nano scale cell in three spatial dimensions of the fast rotating disk. Therefore, after ˜20 years research and development, the super resolution writing disk is still not available.
The six-dimensional storage technology proposed two years ago, however, has a serious shortage. Its data writing and erasing are based on the direct optical linear absorption, that is, the direct single-photon absorption, and its data reading is only through the confocal microscopy filtering. Thus, the interlayer crosstalk noises are large in the data writing and erasing, and not small enough in the data reading, which degrade the system recording and reading qualities seriously. In addition, it is better if its storage capacity can be increased further.